Method of making hemmed joints utilizing laser welding

Abstract

Methods of laser welding zinc coated galvalized steel are disclosed. In a method that relates to forming hem joints for motor vehicle body components a sheet of steel is provided with a plurality of V-shaped tabs. When a hem joint is formed the V-shaped tabs together with a surface of another sheet of steel form gas venting channels that provide an escape passage for gas vapors that are developed when the zinc coating is vaporized by laser welding. In another method of laser welding, one of the steel sheets has an axially extending wall that is V-shaped to provide a V-shaped groove and the wall has spaced gas vent holes.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. A method of making a hem joint between two sheets of steel that have a corrosion protective metallic zinc coating, the steps comprising, forming a first sheet of steel into a configuration that has a hem flange portion which carries a plurality of spaced tab portions that extend from said flange portion, forming each tab portion to define a groove, folding said hem flange portion and said tabs around a marginal portion of a second sheet of steel to form a hem joint, said hem joint being defined by said hem flange portion and a portion of said first sheet with said marginal portion of said second sheet disposed therebetween, the wall of said tab that defines said groove and a portion of said second steel sheet said that faces said wall defining a gas vent channel which is open to the atmosphere at opposite ends of a tab portion, and then directing a laser beam sequentially to each tab portion at a point aligned with a said channel to create a weld zone of molten metal that connects a tab portion to a portion of said second sheet, said channel providing a path for the escape of gas developed by the vaporizing of said zinc coating. 
     
     
       2. The method according to claim 1 where said groove is V-shaped. 
     
     
       3. The method according to claim 1 where said groove has a maximum depth of 0.005 to 0.012 inches. 
     
     
       4. The method according to claim 1 where an adhesive is applied to a surface of said marginal portion of said second sheet prior to said folding of said hem flange, said adhesive after folding providing a sealed hem joint, said tabs being disposed inside of said adhesive. 
     
     
       5. The method according to claim 1 where both sides of said first and second steel sheets are coated with a thin layer of zinc. 
     
     
       6. The method according to claim 1 where the length of said weld zone is such that no weld imperfection is formed on an exterior surface of said first portion of said sheet. 
     
     
       7. The method according to claim 1 where no portion of said weld zone penetrates into said portion of said first sheet. 
     
     
       8. The method of laser welding two sheets of steel that have a corrosion protective metallic zinc coating, the steps comprising, forming a first sheet of steel into a configuration that has a wall defining an axially extending groove, said wall having a plurality of spaced holes extending therethrough defining gas vents, placing said first, sheet of steel against a flat portion of a second sheet of steel whereby a portion of said flat portion that faces said groove and said wall define an axially extending channel that is open at its ends and is open to said spaced holes, and then directing a laser beam to lengths of said wall disposed between said holes to thereby laser weld said lengths to said second sheet of steel, said channel and holes operative to provide paths for the escape of gases developed by vaporization of said zinc coating during laser welding. 
     
     
       9. The method according to claim 8 where the groove is v-shaped.